This invention concerns visual display devices utilizing inorganic phosphors that emit visible electromagnetic radiation when excited by energetic electrons (cathodoluminescence, CL) or by electromagnetic radiation (photoluminescence, PL), and the discussion herein will be so limited.
Although visual display technology utilizing phosphors, especially cathode ray tube (CRT) technology, is the product of many years of intense effort and is quite mature, there exist numerous potential applications for such displays that are currently unrealized or only imperfectly realized. These applications share a common requirement of high phosphor luminosity. Among these applications are projection displays, high resolution displays, and high brightness displays for viewing in the presence of high ambient light levels, e.g., avionic displays, such as head-up displays and flight simulators. Although novel technologies are being developed for these and other applications, e.g., holographic combiners, that require intense narrow-band luminescence emission, the existence of a well-developed CRT display technology based on broadband emission suggests that development of high intensity broadband phosphors might be an economically advantageous approach to the realization of such devices.
Prior art display devices often employ powder phosphors. These phosphors are typically subject to shortcomings such as poor thermal properties, degradation of efficiency with dose, and limited resolution. Many of these shortcomings of prior art powder phosphor targets can be alleviated by the use of single crystal phosphor targets, typically consisting of a monolithic slab of crystalline phosphor that is bonded at the atomic level to a single crystal faceplate. Such epitaxial targets are typically grown by liquid phase epitaxy (LPE). See, for instance, J. M. Robertson et al, Philips Journal of Research, Vol. 35, pp. 354-371, (1980). Single crystal targets have been found to be capable of withstanding input power densities in excess of 10.sup.10 W/m.sup.2 without showing measurable degradation, and can have improved resolution as compared to powder phosphor targets.
The principles of luminescence in inorganic solids are well known and not be reviewed herein in detail. (See, for instance, Luminescence of Inorganic Solids, P. Goldberg, Editor, Academic Press, New York (1966), especially pp. 2-52.) Briefly, luminescent emission from inorganic solids involves optical transitions between electronic states characteristic of the radiating substance. The radiating entity, e.g., an atom occupying a crystal lattice site, is raised into an excited state through interaction with the excitation means, e.g., a UV photon or an energetic electron, followed by the entity's return to the electronic ground state, typically by a series of transitions comprising at least one radiative transition involving emission of a photon of wavelength in the visible part of the spectrum.
Luminescence of most inorganic solids involves impurities, e.g., dopants, or structural defects. If the impurity or defect is the radiation-emitting entity, it is referred to as an "activator," and we will follow this usage herein. The presence of a second species of impurity or defect in the activator-containing host material often affects the emission characteristics of the material. If the second species absorbs energy from the exciting means and transfers part of the energy to the activator, with the reverse energy transfer being small, then the luminescent efficiency of the material is typically enhanced. In such a case, the second species is generally referred to as a "sensitizer," and we will also follow this usage herein.
Although single crystal phosphors tend to have luminescent properties whose gross features are similar to those of powder phosphors of the same composition, the detailed features tend to differ in a generally unpredictable manner. Several reasons exist for this. For instance, since a powder phosphor is typically formed by a different process than the single crystal phosphor, chemical differences (e.g., different phases) may exist. Also, the crystal lattice in powder particles can be expected to be heavily strained, with a high defect density, whereas the lattice of a single crystal phosphor typically is relatively strain free and free of defects. Since luminescence is quite sensitive to the details of the crystal field, these lattice differences can lead to significant differences in the luminescence.
One of the materials whose luminescent properties have been investigated extensively is Y.sub.3 Al.sub.5 O.sub.12, yttrium aluminum garnet (YAG). In particular, rare earth doped (including Ce.sup.3+ as well as Tb.sup.3+ doped) YAG has been investigated. (See, for instance, D. J. Robbins et al, Physical Review B, Vol. 19(2), pp. 1254-1269, (1979).) The energy transfer from different sensitizer species to a variety of activator species was also studied in YAG. In particular, the transfer from Ce.sup.3+ to Tb.sup.3+ in YAG powder has been studied (G. Blasse and A. Brill, The Journal of Chemical Physics, Vol. 47(6), pp. 1920-1926, (1967)). Also, YAG phosphor doped with Ce and Tb was prepared in powder form by calcination. Chemical Abstracts, Vol. 81, 97658d, page 421 (1974) and ibid., 97659e. Japanese Pat. No. 50-97590, Y. Fukuda et al, disclosure date Aug. 2, 1975, also discloses powder YAG phosphor doped with Ce and Tb, and teaches that addition of Tb to Ce-containing YAG resulted in increased brightness. In particular, in indicates that the brightness improvement is due to the additional emission from Tb, and that the emission time of Ce is not increased due to the presence of Tb.
Epitaxially grown monocrystalline Ce- or Tb-doped YAG CRT phosphor screens were also investigated (e.g., J. M. Robertson and M. W. van Tol, Applied Physics Letters, Vol. 37(5), pp. 471-472, (1980)). Saturation effects of the CL in such layers were also determined (e.g., W. F. van der Weg and M. W. van Tol, Applied Physics Letters, Vol. 38(9), pp. 705-707 (1981)).
Because of the great potential of high brightness visual displays, a phosphor with broadband emission that has high conversion efficiency, high power capability, high quench temperature, permits high resolution, and is not subject to substantial degradation is of considerable technological significance. This application discloses such a phosphor.